A. Technical Field
This invention relates generally to electrical device testing and optimization, and more particularly, to the measurement of jitter and the display of an eye diagram during the characterization of an electrical device.
B. Background of the Invention
The importance of electrical device testing and optimization is well known in the art. A number of different technological advancements have improved the performance of communication devices and led to ever-increasing rates at which data may be sent between electrical devices. These advancements have also given rise to a new era of electronic product verification and testing. In particular, the importance of properly measuring signal characteristics, such as a signal-to-noise ratio, jitter, and other signal integrity characteristics, may become even more important as communications systems become increasingly complex.
Electrical components need to be tested and verified in order to ensure that appropriate communication occurs within a system. This component or device characterization ensures proper functioning and compatibility of the devices within a communications system. Device characterization often includes the verification and testing of various signal characteristics including a jitter analysis, timing interval analysis, and eye diagram analysis. This characterization may also include testing the device relative to a simulation of the environment in which the device will operate.
An eye diagram visually presents timing and voltage uncertainty associated with the signal. In particular, an eye diagram is seen by overlaying the multiple edges of the signal across many cycles. Each waveform is overlaid based on a specific reference, usually a clock signal. The thickness of the overlaid signal waveforms represent the timing variations of the signal which is also referred to as jitter.
These variations in signal may in part be attributed to various types of jitter, each of which may be addressed during the characterization procedures, including device testing and optimization. The task of efficiently measuring these different types of jitter, and optimizing the device accordingly, may present difficulties to a test engineer.
The broadest classification of jitter may be described relative to two categories, deterministic jitter and random jitter. Deterministic jitter component comes from the bounded sources such as crosstalk, inter-symbol interference (“ISI”), and power supply feed-through. Since, such a type of jitter is bounded, it is easier to predict and be tested. Comparatively, random jitter is unbounded and much more difficult to characterize. Random jitter may be generated from various sources including thermal noise, shot noise, white noise, etc. and is relatively more difficult to predict. Also, there may exist uncorrelated jitter on a signal, which is caused by non-synchronized noise generated by one or more different clocks within a system.
Deterministic jitter may include a sinusoidal jitter component. To characterize this type of jitter, longer time variations should be observed and used to generate an eye diagram. As described above, the random jitter component may not be easily characterized using an eye diagram. An eye diagram, which tracks some instances of the signal, may fail to catch the random jitter component because only periodic samples of data are collected and the random jitter component may not appear in the eye diagram.
There are devices which enable user to either view random jitter data or an eye diagram. FIG. 1 shows a block diagram representation of a prior art procedure of receiver characterization. An electrical device 102, often referred to a Device Under Test (“DUT”), to be characterized is first connected to an oscilloscope 104 to observe a corresponding eye diagram. As explained above, the eye diagram does not allow efficient monitoring of random jitter component. The eye diagram may be used to determine whether a transmitter, receiver or other device is compliant to a specification; however, it often fails to provide enough information or distinct values for accurate jitter measurements. In particular, the eye diagram provides peak-to-peak measurements and eye opening measures, such as voltage (vertical opening) and timing (horizontal opening).
The electrical device 102 is disconnected from the oscilloscope 104 and connected to a jitter measuring device 106. If the device settings are found not to be optimum during the jitter measuring phase of the device characterization procedure, the whole procedure needs to be repeated by disconnecting the electrical device 102 from the jitter measuring device 106 and reconnected to the oscilloscope 104 with the new settings. This makes the procedure of device characterization cumbersome and error prone since it requires repeatedly attaching and detaching the electrical device 102 to either the oscilloscope 104 or jitter measuring device 106.